Everyone measures drops differently. It is up to you to figure out how many drops it takes you to make 1 ml. Since drops are proportional to the specific gravity of the liquid, it is not uniform over different concentrations or solutions.
On the other hand, 1 ml = 1 cc = 1 ml. If I direct you to mix up a solution containing 1 ml of 28%, or 1 ml of 5%, I know you will get the correct amount, because 1 ml is independent of the specific gravity of the solution.
Since I am concerned with the specific concentration of the mixed solution, accuracy is needed, and ml measurements are more accurate and repeatable than drop measurements.